sorry didn't read the part with the tshirts! so as they are I think the third favours your design best. Now if you lighten the background of the first one I really love the blue hue of the city! so up to you! good luck!

Cool idea but the styles seem to conflict. You have the style of the mouth and then the style of the character, they look foreign together. Use the white outline with the rest of the character. Maybe have some dark red/black around the corners inside the mouth so that its not all red and it looks like it has some depth and begins to show a tongue.

Slate or blue.
The red mouth is a bit strong got me though - either try a different red or maybe a darker grey instead. It you could try the whole thing on a red shirt but I see other people are also suggesting loads of colour ideas and if you try too many it can get confusing!

Perhaps harder edges for the shading in the monster's fur and well defined line-art contouring the little guy's mouth might tie the design together seamlessly. If you follow the strong lines that you have going on with the buildings it may be all that's missing to really pull it together.